CA2965445C

CA2965445C - Transaction messaging

Info

Publication number: CA2965445C
Application number: CA2965445A
Authority: CA
Inventors: Brian Sullivan
Original assignee: Visa Europe Ltd
Current assignee: Visa Europe Ltd
Priority date: 2014-10-24
Filing date: 2015-10-26
Publication date: 2023-03-14
Anticipated expiration: 2035-10-26
Also published as: AU2021203184A1; AU2021203184B2; GB201419016D0; US10769628B2; MX2017005313A; CN107077670B; US20240354756A1; WO2016063089A1; CA3186475A1; CA2965445A1; EP3210177A1; US20170228728A1; US12051064B2; CN113344570A; EP3822891A1; CN107077670A; KR102613422B1; AU2015334634A1; KR20230008206A; AU2015334634B2

Abstract

There is described a method for transmitting a transaction message from a transaction device having a transaction device identifier. The method involves encrypting, at a transaction device, a transaction device identifier, generating, at the transaction device, a transaction message for a transaction system, the transaction message comprising, at least, a first data field configured to hold a transaction device identifier and a second data field configured to hold supplementary data, and sending the transaction message to a transaction processing system. The generation of the transaction message comprises providing data in the first data field of the transaction message that does not identify the transaction device and providing the encrypted transaction device identifier in the second data field of the transaction message.

Description

Transaction Messaging Field of the Invention The present invention relates to systems and methods for transmitting and processing transaction messages, and in particular to systems and methods useable for sending transaction messages where an identifier is encrypted.
Background Smart devices, such as smart cards, access cards, financial instruments such as payment cards, fobs and most recently mobile telephones and other portable electronic devices are increasingly being used to effect transactions. A transaction may involve a number of functions. In a simple form, a user in possession of a suitable smart device may be granted access through a security door. Alternatively, or in addition, such a user may be able to make payments for goods and services, or to use the smart device in ticketing for access to public transport or an event.
A suitable smart device has a processor and a memory. These may be combined in a secure element, which is a piece of tamper resistant hardware which can only be communicated with in a limited fashion.
In use, the smart device is presented to a terminal of a transaction processing system, for example a door lock, a point of sale device or a ticket barrier.
The smart device communicates with the terminal. This communication may be contactless for example using near field communications (NFC), or through contact between the device and the terminal. The smart device may communicate solely with the terminal;
however more often data provided by the smart device is transmitted through the transaction processing system to a suitable recipient. This recipient authenticates the smart device and may respond, for example by commanding the terminal to open a door or barrier, or by providing data to the smart device.
To enable a smart device to be used in this manner, the smart device is provided with a device identifier. This may be a number, or alphanumeric string which is capable of uniquely identifying the device and thereby enabling the transaction processing system to determine whether to grant access, or to effect payment, or similar.
An

2 example of a suitable device identifier used in payments is a primary account number or PAN, which is used on credit and debit cards to effect payments.
Methods have been proposed to modify or obscure a device identifier during a transaction to increase security. While such proposed systems make it harder for a malicious third party to clone or pretend to be the smart device, such systems still do not obviate a risk that a third party may track a user's movements and activity using data transferred from the device.
Summary of the Invention In accordance with at least one embodiment, methods, devices, systems and software are provided for supporting or implementing functionality to transmit and/or process transaction messages.
This is achieved by a combination of features recited in each independent claim.
Accordingly, dependent claims prescribe further detailed implementations of various embodiments.
According to a first aspect of the invention there is provided a method for transmitting a transaction message, the method comprising: encrypting, at a transaction device, a transaction device identifier; generating, at the transaction device, a temporary transaction device identifier; causing a transaction message to be sent to a transaction processing system, the transaction message comprising, at least, a first data field configured to hold a transaction device identifier and a second data field configured to hold supplementary data, wherein the temporary transaction device identifier is provided in the first data field and the encrypted transaction device identifier is provided in the second data field.
The temporary transaction device identifier may be generated at least partly using further data, which further data is other than the transaction device identifier.
Furthermore, the transaction message may comprise a third data field, and data at least partly identifying a cryptographic key associated with the encryption of the transaction device identifier may be provided in the third data field. The cryptographic key may be generated at least partly using further data, which further data is other than the transaction device identifier. The cryptographic key may further be generated using, in addition, a value uniquely associated with the transaction device identifier.

3 The method may comprise receiving the further data from the transaction processing system. For a given transaction, at least part of the further data may be specific to the transaction.
In embodiments, the transaction device identifier may comprise data indicative of a primary account number of a financial instrument.
According to a second aspect of the invention there is provided a method of processing a transaction message, the method comprising: receiving a transaction message from a first part of a transaction processing system, the transaction message comprising, at least, a first data field configured to hold a transaction device identifier and a second data field configured to hold supplementary data, the first data field comprising a first transaction device identifier; decrypting data within the supplementary data field to identify a second transaction device identifier;
processing the transaction message based on the second transaction device identifier.
The processing may comprise sending a modified transaction message with the first transaction device identifier replaced by the second transaction device identifier to a second part of a transaction processing system. The method may further comprise storing an association between the first transaction device identifier and the second transaction device identifier.
The method may comprise: receiving a response message from the second part of the transaction processing system, the response message comprising a data field configured to hold a transaction device identifier, the data field comprising the second transaction device identifier; modifying the response message to replace the second transaction device identifier with the first transaction device identifier;
and sending the modified response message to the first part of the transaction processing system.
The transaction message may comprise a third data field, and data at least partly identifying a cryptographic key associated with the encryption of the data within the second data field may be provided in the third data field. In addition, the transaction message may comprise one or more further data fields configured to hold further transaction data, and the method may comprise verifying the further transaction data using the data in the third data field.
The transaction message may comprise one or more further data fields configured to hold further transaction data, and the method may comprise generating a cryptographic key used to decrypt data within the second data field from the further

4 transaction data. The method may comprise using, in addition, a value uniquely associated with the second transaction device identifier, to verify the further transaction data.
In embodiments, the transaction device identifier may comprise data indicative of a primary account number of a financial instrument.
According to a third aspect of the invention there is provided a method for transmitting a transaction message, the method comprising: generating, at a transaction device, at least one cryptographic data element using at least a transaction device identifier and further data other than the transaction device identifier as inputs to an cryptographic function; causing a transaction message to be sent to a transaction processing system, the transaction message comprising, at least, the further data and the at least one cryptographic data element.
The at least one cryptographic data element may comprise an encrypted version of the transaction device identifier. Furthermore, the method may comprise:
generating a cryptographic key using at least the further data; and generating the encrypted version of the transaction device identifier using the cryptographic key. The method may further comprise using, in addition, a value uniquely associated with the transaction device identifier to generate the cryptographic key. The at least one cryptographic data element may further comprise data at least partly identifying the cryptographic key.
In embodiments, the transaction device identifier may comprise data indicative of a primary account number of a financial instrument.
According to a fourth aspect of the invention there is provided a method of processing a transaction message, the method comprising: receiving, from a first part of a transaction processing system, a transaction message, the transaction message comprising a first transaction device identifier and at least two cryptographic data elements; and using a first of the cryptographic data elements to determine a cryptographic key; decrypting a second of the cryptographic data elements whereby to determine a second transaction device identifier; and processing the transaction message based on the second transaction device identifier.
The processing may comprise sending a modified transaction message with the first transaction device identifier replaced by the second transaction device identifier to a second part of a transaction processing system. The method may also comprise storing an association between the first transaction device identifier and the second transaction device identifier.
The method may comprise: receiving a response message from the second part of the transaction processing system, the response message comprising a data field

5 configured to hold a transaction device identifier, the data field comprising the second transaction device identifier; modifying the response message to replace the second transaction device identifier with the first transaction device identifier;
and sending the modified response message to the first part of the transaction processing system.
The transaction message may comprise one or more further data fields configured to hold further data, and the method may comprise verifying the further data using the first of the cryptographic data elements. The method may further comprise using, in addition, a value uniquely associated with the second transaction device identifier, to verify the further data.
In embodiments, the transaction device identifier may comprise data indicative of a primary account number of a financial instrument.
According to a fifth aspect of the invention there is provided a method of processing a transaction message, the method comprising: receiving, from a first part of a transaction processing system, a transaction message, the transaction message comprising a first transaction device identifier, a cryptographic data element and further data; generating a cryptographic key using at least the further data;
decrypting the cryptographic data element using the cryptographic key whereby to determine a second transaction device identifier; and processing the transaction message based on the second transaction device identifier.
According to a sixth aspect of the invention there is provided apparatus for use in transmitting a transaction message from a transaction device, the apparatus configured to: encrypt a transaction device identifier; generate a temporary transaction device identifier; cause a transaction message to be sent to a transaction processing system, the transaction message comprising, at least, a first data field configured to hold a transaction device identifier and a second data field configured to hold supplementary data, wherein the temporary transaction device identifier is provided in the first data field and the encrypted transaction device identifier is provided in the second data field.
According to a seventh aspect of the invention there is provided apparatus for processing a transaction message, the apparatus configured to: receive a transaction

6 message from a first part of a transaction processing system, the transaction message comprising, at least, a first data field configured to hold a transaction device identifier and a second data field configured to hold supplementary data, the first data field comprising a first transaction device identifier; decrypt data within the supplementary data field to identify a second transaction device identifier; process the transaction message based on the second transaction device identifier.
According to a eighth aspect of the invention there is provided apparatus for use in transmitting a transaction message from a transaction device, the apparatus configured to: generate at least one cryptographic data element using at least a transaction device identifier and further data as inputs to an cryptographic function;
cause a transaction message to be sent to a transaction processing system, the transaction message comprising, at least, the further data and the at least one cryptographic data element.
According to a ninth aspect of the invention there is provided apparatus for processing a transaction message, the apparatus configured to: receive, from a first part of a transaction processing system, a transaction message, the transaction message comprising a first transaction device identifier and at least two cryptographic data elements; and use a first of the cryptographic data elements as a cryptographic key to decrypt a second of the cryptographic data elements whereby to determine a second transaction device identifier; and process the transaction message based on the second transaction device identifier.
According to a tenth aspect of the invention there is provided apparatus for processing a transaction message, the apparatus configured to: receive, from a first part of a transaction processing system, a transaction message, the transaction message comprising a first transaction device identifier, a cryptographic data element and further data; generate a cryptographic key using at least the further data; decrypt the cryptographic data element using the cryptographic key whereby to determine a second transaction device identifier; and process the transaction message based on the second transaction device identifier.
According to a eleventh aspect of the invention there is provided a computer program for use in transmitting a transaction message from a transaction device, the computer program configured to: encrypt a transaction device identifier;
generate a temporary transaction device identifier; cause a transaction message to be sent to a

7 transaction processing system, the transaction message comprising, at least, a first data field configured to hold a transaction device identifier and a second data field configured to hold supplementary data, wherein the temporary transaction device identifier is provided in the first data field and the encrypted transaction device identifier is provided in the second data field.
According to a twelfth aspect of the invention there is provided a computer program for processing a transaction message, the computer program configured to:
receive a transaction message from a first part of a transaction processing system, the transaction message comprising, at least, a first data field configured to hold a transaction device identifier and a second data field configured to hold supplementary data, the first data field comprising a first transaction device identifier;
decrypt data within the supplementary data field to identify a second transaction device identifier;
process the transaction message based on the second transaction device identifier.
According to a thirteenth aspect of the invention there is provided a computer program for use in transmitting a transaction message from a transaction device, the computer program configured to: generate at least one cryptographic data element using at least a transaction device identifier and further data as inputs to an cryptographic function; cause a transaction message to be sent to a transaction processing system, the transaction message comprising, at least, the further data and the at least one cryptographic data element.
According to a fourteenth aspect of the invention there is provided a computer program for processing a transaction message, the computer program configured to:
receive, from a first part of a transaction processing system, a transaction message, the transaction message comprising a first transaction device identifier and at least two cryptographic data elements; and use a first of the cryptographic data elements as a cryptographic key to decrypt a second of the cryptographic data elements whereby to determine a second transaction device identifier; and process the transaction message based on the second transaction device identifier.
According to a fifteenth aspect of the invention there is provided a computer program for processing a transaction message, the computer program configured to:
receive, from a first part of a transaction processing system, a transaction message, the transaction message comprising a first transaction device identifier, a cryptographic data element and further data; generate a cryptographic key using at least the further

8 data; decrypt the cryptographic data element using the cryptographic key whereby to determine a second transaction device identifier; and process the transaction message based on the second transaction device identifier.
According to a sixteenth aspect of the invention, there is provided a method for transmitting a transaction message from a transaction device having a transaction device identifier, the method comprising: encrypting, at a transaction device, a transaction device identifier; generating, at the transaction device, a transaction message for a transaction system, the transaction message comprising, at least, a first data field configured to hold a transaction device identifier and a second data field configured to hold supplementary data; and sending the transaction message to a transaction processing system, wherein the generation of the transaction message comprises providing data in the first data field of the transaction message that does not identify the transaction device and providing the encrypted transaction device identifier in the second data field of the transaction message.
According to a seventeenth aspect of the invention, there is provided a method of processing a transaction message, the method comprising: receiving a transaction message from a first part of a transaction processing system, the transaction message comprising, at least, a first data field configured to hold a transaction device identifier and a second data field configured to hold supplementary data; decrypting data within the supplementary data field to generate decrypted data; processing the transaction message using said decrypted data to form the transaction device identifier instead of the data in the first data field.
According to an eighteenth aspect of the invention, there is provided apparatus for use in transmitting a transaction message from a transaction device, the apparatus configured to: encrypt a transaction device identifier; generate a transaction message comprising, at least, a first data field configured to hold a transaction device identifier and a second data field configured to hold supplementary data; and send the transaction message to the transaction processing system, wherein the apparatus is arranged to generate the transaction message such that data that does not identify the transaction device is provided in the first data field and the encrypted transaction device identifier is provided in the second data field.
According to a nineteenth aspect of the invention, there is provided apparatus for processing a transaction message, the apparatus configured to: receive a transaction

9 PCT/GB2015/053200 message from a first part of a transaction processing system, the transaction message comprising, at least, a first data field configured to hold a transaction device identifier and a second data field configured to hold supplementary data; decrypt data within the supplementary data field to generate decrypted data; and process the transaction message using said decrypted data to form the transaction device identifier instead of the data in the first data field.
Further features and advantages will become apparent from the following description of preferred embodiments, given by way of example only, which is made with reference to the accompanying drawings.
Brief Description of the Drawings Systems, apparatuses and methods will now be described as embodiments, by way of example only, with reference to the accompanying figures in which:
Figure 1 shows a schematic diagram of a transaction system in which embodiments of the invention may be practised;
Figure 2 illustrates a communication flow according to an embodiment;
Figure 3 illustrates a method according to an embodiment;
Figure 4 illustrates a further method according to an embodiment; and Figure 5 shows a schematic diagram of a device which may be used in embodiments of the invention.
Some parts, components and/or steps of the embodiments appear in more than one Figure; for the sake of clarity the same reference numeral will be used to refer to the same part, component or step in all of the Figures.
Detailed Description of Illustrative Embodiments Figure 1 shows a transaction system 10. A transaction device 12, is provided.
Examples of suitable transaction devices include smart cards, access cards, fobs, financial instruments such as payment cards, mobile telephones and other portable electronic devices such as tablets and smart watches. The transaction device 12 has a data connection to a transaction terminal 14. Examples of suitable transaction terminals include payment terminals, access points to transit systems, and doors.
The data connection between the transaction device 12 and the transaction terminal 14 may be contactless. Examples of contactl es s connection technologies which may be used include near field communications (NFC) and optical systems ¨ the latter being, for example, provided by a system which uses a camera in a mobile telephone to identify and read data presented on e.g. a screen of the terminal. The data connection may alternatively be a contact connection using a suitable arrangement of electrically 5 conductive pads and pins to enable communication.
The transaction terminal 14 is connected to a first transaction processing server 16, which is in turn connected to a second transaction processing server 18.
Together the terminal 14 and servers 16 and 18 may be considered to constitute a transaction processing system 20. While not shown, one or more additional transaction processing

10 servers may be provided between the transaction terminal 14 and first transaction processing server 16. Likewise, one or more additional transaction processing servers may be provided between the first transaction processing server 16 and the second transaction processing server 18. Collectively, the transaction terminal 14 and any additional transaction processing servers between the transaction terminal 14 and first transaction processing server 16 may be considered as a first part of the transaction processing system 20. Equally, the second transaction processing server 18 and any additional transaction processing servers between the first transaction processing server 16 and the second transaction processing server 18 may be considered as a second part of the transaction processing system 20.
While only a single instance of each of the device 12, terminal 14 and servers 16 and 18 are shown, it will be appreciated that the transaction system 10 may be substantially more complex, with multiple devices 12 (representing devices provided to multiple users), multiple terminals 14 (representing, for example, multiple payment terminals or access terminals) and even multiple servers 16 and 18.
The operation of the transaction system 10 shown in Figure 1 during a transaction will now be described with reference to Figure 2. In general, in this transaction a transaction device 12 is presented to a transaction terminal 14, and the transaction processing system 20 operates to authorise or deny the transaction.
Depending on whether the transaction is authorised or denied, the transaction terminal, or a device connected thereto, may perform some action (such as opening a door), alternatively or additionally, a message may be sent back to the transaction device. A
more detailed description follows.

11 In a first step 22, a transaction is instigated and the transaction device 12 connects to the transaction terminal 14. The instigation of the transaction may, for example, include a user selecting goods or services to purchase, or selecting a destination for a ticketing transaction. This may require user input, or alternatively may be predetermined based on the identity of the transaction terminal ¨ for example an identity of a transaction terminal on a transit system may be used to define the service required without any specific user input.
The connection between the transaction device 12 and the transaction terminal 14 may be established by the transaction device 12 being presented to the terminal and a contactless, e.g. near field communication (NFC), connection being established.
Alternatively, a transaction device 12 may be physically inserted into or connected to the transaction terminal 14 to enable an electrical connection to be established. Such methods are known in the art and need not be described in detail here.
Having, in step 22, instigated the transaction and established the connection transaction data associated with the transaction may be sent, in step 24, from the transaction terminal 14 to the transaction device 12. This transaction data may include, for example, a price to be paid in the transaction or an identity of an entry or egress point for a ticketing transaction on a transit system. In general, the nature of the transaction as described above will define the transaction data.
In step 26, the transaction device prepares a transaction message, and in step the transaction message is sent to from the transaction device to the transaction terminal 14. A more detailed description of the content of this message, and the methods by which it is created in steps 24 to 28 will be provided below with reference to Figure 3.
In step 30 transaction terminal then forwards the message on to the first transaction processing server 16. As will be appreciated from the description above, this may involve sending the message via one or more further transaction processing servers.
In step 32, the first transaction processing server 16 processes the transaction message. In some embodiment the first transaction processing server 16 may be capable of authorising or denying the transaction. In such cases, the signalling flow may pass straight to step 40 described below.
In the alternative, the second transaction processing server 18 may be the entity capable of authorising or denying the transaction. In such cases, the first transaction

12 processing server 16 may modify the message. The modified message may then be sent to the second transaction processing server 18 in step 34.
The second transaction processing server 18 then authorises or denies the transaction and, in step 38, sends a response message to the first transaction processing server 16. This response message may again be processed by the first transaction processing server 16, before the modified response message is sent, in step 40, to the transaction terminal 14. A more detailed description of the processing of the message by the first transaction processing server 16 in steps 30 to 40 will be provided below with reference to Figure 4.
Upon receipt of the response message in step 40, the transaction terminal 14 may perform any number of actions. For example, the transaction terminal 14 may send a response message to the transaction device 42. This response message may contain data indicative of the transaction being authorised, and may, if required, include a ticket or other data structure which may be stored by the transaction device 12 for later use.
Alternatively, or additionally, the transaction device may take an appropriate action, shown by step 44. This action may be, for example, to open a door or ticketing barrier, or may be the provision of an indication that the transaction has been authorised (and therefore that the user may be provided with purchased goods or services).
The above processing flow is known in the art of transaction systems and therefore has been described in overview only.
As mentioned above, a more detailed description of the operation of the transaction device in steps 24 to 28 will now be provided with reference to Figure 3.
Here the transaction device will be assumed to be provided with a transaction device identifier. This is a value or code which enables the transaction processing system to identify the transaction device 12 and distinguish it from other similar transaction devices. An example of a suitable transaction device identifier is a primary account number (PAN) of a financial instrument. In addition to the transaction device identifier, authentication data may be stored by the transaction device 12. This authentication data may be used to enable messages sent from the transaction device 12 to be authenticated by the transaction processing system, thereby enabling the transaction processing system to authorise or deny any given transaction. The authentication data may include supplementary credentials and cryptographic keys which have been earlier provided to the transaction device 12.

13 In step 24, as mentioned above, the transaction device 12 may receive transaction data from the transaction processing system. This transaction data may include data associated with the identity of the transaction terminal 14, for example an identity of a merchant or transit services provider which provides or uses that terminal, an identity of the terminal itself, a location for the terminal, a channel or domain associated with the communication with the terminal (this may indicate whether wireless or electrical contact is used), and payment details to enable the merchant to receive payment. In addition, the transaction data may include data which is specific to the transaction itself, for example data indicative of a time for the transaction, an amount for a payment, an amount for a reduction in a pre-paid ticket, and/or an identification of any goods or services associated with the transaction. The transaction data comprises at least some data which is other than the transaction device identifier.
In step 26A, the transaction device 12 generates a cryptographic key using the received transaction data. Typically, the cryptographic key will be generated using the transaction data as an input to one or more cryptographic functions. The transaction data may not be the only input to the function, and the following additional inputs may be used:
- a value uniquely associated with the transaction device identifier, for example a hashing key or seed value stored on the transaction device 12;
the transaction device identifier (it will be apparent that the transaction data referred to above represents data other than the transaction device identifier);
- a value indicative of the number of the transactions which have been previously performed or completed by the transaction device 12; and data indicative of a channel or domain used for the transaction.
One example of how a cryptographic key may be generated will now be described. This example will use Elliptic Curve Cryptography (ECC) and a method called ECC El Gamal for key agreement. It will be assumed that a cyclic group G has been defined based on a generator value g. A public key Ps for a transaction processing server, e.g. server 16 has been generated based on the group G and a private key ds for that server. For example:
Ps = ds = G = gds

14 This public key has been made available to the transaction device. In addition, a further key K, the hashing key, has been defined and is known to both the transaction device 12 and to the first transaction processing server 16. The hashing key K
may be a value uniquely associated with the transaction device.
In a first step, the transaction device 12 calculates a hash value. This may be done using a keyed-hash message authentication code (HMAC). The inputs to the hash function include the hashing key K and, in this example, a concatenation of the device identifier (ID) and the transaction data. The output of the hashing function is denoted by h, and can be written as:
h = HMAC(K, IDI I transaction data) Using h and the cyclic group G the transaction device 12 may generate an ephemeral public key PD for the device for use in the transaction. This public key PD
represents the cryptographic key described above. For example:
PD = h = G = gh In addition, using h and the public key Ps of the first transaction processing server 16, the transaction device 12 may generate a shared secret S. For example:
S = h = Ps = Psh = gdsh Having generated the ephemeral cryptographic key PD, and from that computed the shared secret S, in step 26B the transaction device 12 encrypts the device identifier ID using the shared secret S to generate an encrypted transaction device identifier C. For example:
C = enc(S, /D) The cryptographic key PD, and the encrypted transaction device identifier C
may each be considered cryptographic data elements which may be sent to the transaction terminal 14 in a transaction message.
In addition to generating the encrypted transaction device identifier C, in step 5 26C the transaction device 12 may generate a temporary transaction device identifier.
The temporary transaction device identifier may be generated entirely randomly, or pseudo randomly. Alternatively it may be based on the encrypted transaction device identifier C; or generated using further data, at least some of which is data other than the transaction device identifier, for example the transaction data described above, or 10 the public key PD generated for the device.
The temporary transaction device identifier may be generated using a further function, with one of the values described above as an input. For instance, it is typically the case that the device identifier has a certain format ¨ for example being of a certain length. In such cases, the input value may be modified to provide a temporary

15 transaction device identifier. The temporary transaction device identifier may not be wholly generated, and may be based in part on predetermined data, such as a portion of the real transaction device identifier.
As an example, where the transaction device identifier is a 16 digit PAN, the first 6 digits represent a Bank Identification Number (BIN) or Issuer identification number (TIN), and the last digit represents a check digit. The BIN/TIN from the original transaction device identifier may be kept, and augmented with nine digits of the temporary transaction device identifier and a suitable check digit.
In step 26D, the transaction device 12 creates a transaction message to be sent to a transaction processing system using the values generated above. It will often be the case that the transaction message must conform to a certain standard. For example, the transaction message may be formatted in accordance with an EMV standard for payment processing, which specifies mandatory data elements for the transaction message including a data element configured to convey the PAN as a transaction device identifier. Accordingly this standard may specify that the message should comprise, at least, a first data field configured to hold a transaction device identifier and a second data field configured to hold supplementary data. Accordingly, the temporary transaction device identifier, that does not in fact identify the transaction device 12, may be provided in the first data field and the encrypted transaction device identifier

16 may be provided in the second data field. In addition the transaction message may comprise a third data field, and the cryptographic key PD associated with the encryption of the transaction device identifier may be provided in the third data field.
It will be understood that the cryptographic key PD was not itself used in the encryption of the transaction device identifier. Instead, by virtue of being the public key associated with the shared secret used in the encryption, PD represents data identifying the cryptographic key associated with the encryption of the transaction device identifier.
Finally, some or all of the transaction data may be provided in other fields of the message.
Having generated a suitable transaction message, the transaction device 12, in step 28, sends the transaction message to the transaction processing system, i.e. the transaction terminal 14.
A more detailed description of the operation of the first transaction processing server 16 in steps 30 to 40 will now be provided with reference to Figure 4.
In step 30, the first transaction processing server 16 receives the transaction message. In line with the description above, the message created by the transaction device 12 comprises a temporary transaction device identifier, an encrypted transaction device identifier C and the ephemeral cryptographic key PD. The encrypted transaction device identifier C and the ephemeral cryptographic key PD may be considered as cryptographic data elements. In addition, the message may contain at least some of the transaction data.
In step 32A, the first transaction processing server 16 may generate the shared secret S using the cryptographic key PD. For example:
S = ds = PD = PDdS = ghdS
The shared secret S may then be used to decrypt the encrypted transaction device identifier C to generate the original transaction device identifier ID.
In addition, the first transaction processing server 16 may validate any transaction data provided in the transaction message. This may be done by using the original transaction device identifier ID to look up the hashing key K for the transaction device 12 and then recreating the hash value h' and the ephemeral public key P'D as described above. For example:

17 = HMAC(K, IDI I transaction data) PI D = If = G =
A comparison of the public key PD sent in the transaction message and the newly generated public key P'D will demonstrate whether the transaction data received in the message corresponds to the transaction data used to generate the public key PD.
The first transaction processing server 16 may then process the transaction message based on the original transaction device identifier (i.e. at least part of the decrypted data derived from the encrypted data provided in the second data field). In other words, the transaction message may be processed as if the temporary transaction device identifier were replaced by the original transaction device identifier ID. As mentioned above, the first transaction processing server 16 itself may be able to authorise or deny the transaction at this point. If this is the case, then in step 32E, the first transaction processing server 16 determines, using the original transaction device identifier ID whether to authorise or deny the transaction and generates a suitable response message. In step 40 the first transaction processing server 16 then sends the response message back to the transaction terminal 14.
However, in the alternative the first transaction processing server 16 may, in step 32C, modify the transaction message, replacing the temporary transaction device identifier with the original transaction device identifier ID. The first transaction processing server 16 may additionally, in step 32D, store an association between the temporary transaction device identifier and the original transaction device identifier ID.
Subsequently, in step 34 the first transaction processing server 16 may send the modified transaction message with the temporary transaction device identifier replaced by the original transaction device identifier the second transaction processing server 18.
The second transaction processing server 18 may then process the modified transaction message as a normal message which had been originally provided with an unencrypted transaction device identifier.
In step 36 the first transaction processing server 16 may receive a response message from the second transaction processing server 18. This response message may

18 comprise a data field configured to hold a transaction device identifier, which consequently comprises the original transaction device identifier ID.
In step 38, the first transaction processing server 16 may modify the response message to replace the original second transaction device identifier ID with the first transaction device identifier, using the association stored in step 32D. The modified response message may then, in step 40, be sent to the transaction terminal 14.
The above described methods present the following advantages. Firstly, the field designed to contain the transaction device identifier in a typical transaction system is limited in size and needs to adhere to strict formatting rules. This puts restrictions on the degree of freedom for any temporary transaction device identifier. By providing a temporary transaction device identifier in a message and separately providing an encrypted transaction device identifier, the degree of freedom for encrypting the transaction device identifier is increased, and therefore security is correspondingly increased. Equally, it is easier to generate the temporary transaction device identifier as it only needs to conform to the requirements of being random, or pseudo random, and enabling the transaction message to be properly handled by the transaction system.
Furthermore, by providing the ephemeral cryptographic key with the transaction message, it is possible to ensure that no information provided in the transaction message can be used to track a user. This is because the ephemeral cryptographic key is itself non deterministic or random and therefore cannot be used to track a user.
It should be noted that a system may be used where a transaction device identifier is encrypted using solely a public key of a recipient server. The disadvantage of such system is that they are susceptible to attack, as the relatively static key (that of the server) means that multiple messages are all sent using the same key, which in turn reduces the security of the system.
A further advantage relates to the size of a cryptographic key which is required to enable effective encryption of the transaction device identifier. For example, a typical length of an ECC cryptographic key required to provide adequate encryption is 32 bytes or more. Providing this key in a message takes up a large amount of the message data, often significantly more than the transaction device identifier itself For example, a PAN may be uniquely identified by less than 8 bytes of data, a quarter of the data size of the key which may be used to encrypt the PAN. However, in embodiments the cryptographic key also serves as data enabling the transaction data to be verified. This

19 dual use improves the data size efficiency of any message and enables messages, encrypted according to the embodiments described above, to be transmitted using existing systems with restrictions on the size of any message.
Alternative Embodiments and Modifications While a specific implementation of ECC cryptography has been described above, it will be appreciated that modifications may be made, or entirely different systems may be used, for the generation and use of the cryptographic key. For example, the shared secret S may be used directly to encrypt the device identifier, however in the alternative, a further key, generated using S may be used. Alternatively, an implementation may use lattice based cryptographic methods such as NTRU. For such an implementation, there would be no need to communicate a separate ephemeral key to the server since the output of the encryption mechanism is effectively random.
In the specific implementation described above, the temporary transaction device identifier is random so that no information in the transaction message can be used to track the transaction device 12, and accordingly potentially track the user of the transaction device 12. Alternatively, instead of a random temporary transaction device identifier, a fixed number stored by the transaction device 12 that is the same for many or all transaction devices utilising the invention could be inserted in the field in the transaction message for the transaction device identifier. In this way, it is not possible to determine the identity of the transaction device from the entry within the transaction device identifier field within the transaction message. Further, such a static transaction device identifier can be used to indicate to a recipient of the transaction message that an encrypted version of the actual transaction device identifier is provided in a separate field of the transaction message.
Embodiments are intended to be compatible with existing systems. Therefore the message sent by the transaction device 12 may conform to existing protocols. In particular, it is intended that only the transaction device 12 and the first transaction processing server 16 need be modified to enable the overall system to operate as before.
Consequently, the first transaction processing server 16 may operate to convert any message provided by the transaction device 12 into a format which is usable by the second transaction processing server 18 without requiring modification of the second transaction processing server 18.

In some embodiments, the temporary transaction device identifier may correspond to the encrypted transaction device identifier, and may therefore be used to retrieve the original device identifier. This obviates the need for any further field.
While the cryptographic key has been described as being transmitted with the 5 transaction message, some embodiments may be arranged to generate the cryptographic key from the transaction data in a manner which can be replicated by the first transaction processing server 16. Accordingly, at the first transaction processing server 16, a cryptographic key may be generated using at least the transaction data provided in the transaction message. This cryptographic key may then be used to decrypt any 10 cryptographic data element containing the original transaction device identifier.
In embodiments, the original transaction device identifier may be a pointer to a transaction device identifier useable by the second transaction processing server 18. As such the first transaction processing server 16 may possess a lookup table which enables the pointer to be used to identify a suitable transaction device identifier.
Therefore, no 15 modification is needed for the second transaction processing server 18, but the identifier passed between the transaction device 12 and the first transaction processing server 16 need not be selected according to the requirements for a suitable identifier useable by the second transaction processing server 18.
In the specific implementation described above, the transaction terminal 14

20 forwards the transaction message from the transaction device 12 to a first transaction processing server 16 and the first transaction processing server 16 recovers the transaction device identifier for the transaction device 12. In some implementations, the transaction terminal 14 may be required to authorise a transaction faster than can be achieved if the transaction terminal awaits a response from the first transaction processing server. An example of such an implementation is a ticket gate arrangement in which a transaction terminal within a ticket gate needs to authorise a transaction and transfer ticket information to a transaction device 12 within a short time. In such an implementation, the transaction terminal 14 may send a message to the transaction device 12 including a public key certificate for the transaction terminal 14, and the transaction device 12 may encrypt the transaction device identifier using the public key derived or extracted from the certificate for the transaction terminal 14, preferably using a cryptographic scheme as described above, and send the encrypted transaction device identifier to the transaction terminal 14. The transaction terminal 14 may then recover

21 the transaction device identifier and, for example, compare the recovered transaction device identifier with a blacklist of transaction device identifiers stored by the transaction terminal 14, indicating transaction devices 12 for which no transaction should be made, before authorising the transaction.
In embodiments, the transaction device itself may be a self-contained device, such as a smart card or fob. In other embodiments, the transaction device 12 be a general purpose computing device, such as a mobile phone or computer, which is contains, or is connected to apparatus which generates the transaction messages. Such an apparatus may be tamper resistant hardware; that is a secure element. In such cases, it will be appreciated that reference to the transaction device performing a given operation, such as sending a message to a terminal, is representative of the transaction device causing another device (e.g. the mobile telephone) to send such a message.
Recently, systems whereby a computing device (such as a mobile telephone) can be used without requiring a secure element, have been proposed. On such system is called "Host Card Emulation" whereby a transaction application executes within a device's application processor. An alternative, but similar system is the use of a "Trusted Execution Environment" within a suitable device. Embodiments of the invention are applicable to these and similar systems.
In some embodiments, the transaction device 12 may not receive any transaction data from the transaction terminal 14, but may generate the transaction data itself In further embodiments the transaction data may be received by other means. For example it has been proposed to use mobile telephones in transactions, and to enable those mobile telephones to send and receive data via the mobile network alongside any transaction which may occur over a contactless (e.g. NFC) connection between the mobile telephone and a terminal. In such cases, it is envisaged that at least some of the communications described above, whether the provision of transaction data to the transaction device, or transmission of the transaction message, may not involve the transaction terminal 14, but other communications systems.
The connection between the transaction device 12 and the transaction terminal 14 may be bidirectional as described above, but may equally be unidirectional.
For example, a mobile telephone may receive transaction data from a terminal via a unidirectional connection (e.g. by photographing a code optically displayed by the terminal) and may then create and send a suitable transaction message via a wireless

22 communications network such as a cellular connection or WiFi. In such cases the transaction terminal may not itself have any communications capabilities with the transaction processing network, and may be, for example, a poster displaying an optical code such as a QR code.
In other embodiments, there may not be a transaction terminal 14 as such, and the transaction device 12 may communicate directly with a network and thereby with the first transaction processing server 16. This may be used for online transactions where the transaction device 12 is a connected computer or portable device.
In some embodiments, other information, such as credentials for enabling the message to be authenticated, may be encrypted alongside the device identifier.
The cryptographic key may be compressed. For example a full elliptical function cryptographic key has an X and a Y component. It is possible to compress the key be providing only the X component alongside one or two bits of data to indicate a sign for the Y component. Knowledge of the X component, the function used, and the sign of the Y component enables the cull cryptographic key to be recreated. In the above description it will be appreciated that where a key is described as being determined or provided, a compressed version thereof may be equivalently used.
The transaction device 12, transaction terminal 14 and transaction processing servers 16 and 18, may comprise computerised hardware as is known in the art.
An exemplary computerised system 50, capable of performing the method steps described above, will now be described with reference to Figure 5.
The computerised system 50 comprises a processing system 51, such as a CPU, or an array of CPUs. The processing system 51 is connected to a computer readable storage medium such as memory 52. This memory may be a volatile memory, for example RAM; or a non-volatile or non-transitory memory, for example a solid state drive (SSD) or hard disk drive (HDD). The system 50 may also comprise an interface 54, capable of transmitting and/or receiving data from other elements in the system.
The memory 52 stores computer readable / computer executable instructions 53.
The computer readable instructions may be configured such that when they are executed by the processing system 51, the computerised system 50 is caused to perform the methods described above. To enable this, the processing system 51 may retrieve the computer instructions 53 from memory 52 and execute these instructions. In so doing, the processing system 51 may cause the interface to transmit or receive data as required.

23 This data may itself be stored in memory 52, and retrieved as required ¨ for example to be transmitted via the interface 54.
It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.
The features of the claims may be combined in combinations other than those specified in the claims.

Claims

1. A method for transmitting a transaction message from a transaction device having a transaction device identifier ID to a transaction processing system, wherein the transaction message conforms to an EMV standard for payment processing in which the transaction message comprises, at least, a first data field configured to hold the transaction device identifier, a second data field configured to hold supplementary data, a third data field, and other data fields for conveying transaction data, the method comprising:
receiving, at the transaction device, transaction data from a transaction processing server of the transaction processing system;
receiving, at the transaction device, a public key Ps for the transaction processing server generated based on a cyclic group G and a private key ds for the transaction processing server;
calculating, at the transaction device, a hash value h by inputting a hashing key K, known to both the transaction device and the transaction processing server, and a concatenation of the transaction device identifier and the transaction data for a transaction;
generating, at the transaction device, an ephemeral public key PD for the transaction device using the hash value h and the cyclic group G;
generating, at the transaction device, a shared secret S using the hash value h and a public key Ps for the transaction processing server;
encrypting, at the transaction device, the transaction device identifier ID
for the transaction device using the shared secret S to generate an encrypted transaction device identifier C;
generating, at the transaction device, the transaction message for the transaction processing system; and sending the transaction message to the transaction processing system, wherein the generation of the transaction message comprises providing data in the first data field of the transaction message that does not identify the transaction device, providing the encrypted transaction device identifier C in the second data field of the transaction message, providing the ephemeral public key PD in the third data field.

2. The method of claim 1, wherein the ephemeral public key PD is generated using, in addition, a value uniquely associated with the transaction device identifier ID.

3. The method of claim 1, further comprising generating, at the transaction device, a temporary transaction device identifier at least partly using further data, which further data is other than the transaction device identifier, wherein said data provided in the first field is the temporary transaction device identifier.

4. The method of claim 1, wherein the transaction device identifier ID
comprises data indicative of a primary account number of a financial instrument.

5. A method of processing a transaction message from a transaction device, wherein the transaction message conforms to an EMV standard for payment processing in which the transaction message comprises, at least, a first data field configured to hold a transaction device identifier, a second data field configured to hold supplementary data, a third data field, and other data fields conveying transaction data, the method comprising:
receiving, at a transaction processing server, the transaction message from a first part of a transaction processing system;
generating, at the transaction processing server, a shared secret S using a private key ds for the transaction processing server and an ephemeral public key PD
stored in the third data field;
decrypting an encrypted transaction device identifier C stored in the second data field using the shared secret S to generate an original transaction device identifier ID;
processing the transaction message using the generated original transaction device identifier ID to form the transaction device identifier instead of the data in the first data field; and validating the transaction data using the generated original transaction device identifier ID, wherein validating the transaction data comprises:
looking up a hashing key K corresponding to the transaction device identifier ID;

obtaining a recreated hash value h' by inputting the hashing key K and a concatenation of the generated original transaction device identifier ID and the transaction data for a transaction and obtaining a recreated ephemeral public key P 'D using the recreated hash value h' and a cyclic group G; and comparing the ephemeral public key PD stored in the third data field and sent in the transaction message and the recreated ephemeral public key P 'D.

6. The method of claim 5, wherein the processing comprises sending a modified transaction message with the data in the first data field replaced by said generated original transaction device identifier ID to a second part of a transaction processing system.

7. The method of claim 6, comprising storing an association between said data in the first data field and said generated original transaction device identifier b;
receiving a response message from the second part of the transaction processing system, the response message comprising another data field holding the generated original transaction device identifier b;
modifying the response message to replace the generated original transaction device identifier ID with said data in the first data field; and sending the modified response message to the first part of the transaction processing system.

8. The method of claim 5, wherein the transaction device identifier ID
comprises data indicative of a primary account number of a financial instrument.

9. A method for transmitting a transaction message from a transaction device having a transaction device identifier ID to a transaction processing system, the method comprising:
receiving, at the transaction device, transaction data for a transaction;
receiving, at the transaction device, a public key Ps for the transaction processing server generated based on a cyclic group G and a private key ds for the transaction processing server;

calculating, at the transaction device, a hash value h by inputting a hashing key K, known to both the transaction device and the transaction processing server, and a concatenation of the transaction device identifier and the transaction data;
generating, at the transaction device, an ephemeral public key PD for the transaction device using the hash value h and the cyclic group G;
generating, at the transaction device, a shared secret S using the hash value h and a public key Ps for the transaction processing server;
encrypting, at the transaction device, the transaction device identifier ID
for the transaction device using the shared secret S to generate an encrypted transaction device identifier C;
sending the transaction message to the transaction processing system, the transaction message comprising, at least, the ephemeral public key PD and the encrypted transaction device identifier C.

10. The method of claim 9, comprising using, in addition, a value uniquely associated with the transaction device identifier to generate the ephemeral public key PD.

11. The method of claim 10, wherein the at least one cryptographic data element further comprises data at least partly identifying the ephemeral public key PD.

12. The method of any one of claims 9 to 11, wherein the transaction device identifier comprises data indicative of a primary account number of a financial instrument.

13. A method of processing a transaction message from a transaction device, the method comprising:
receiving, at a transaction processing server, the transaction message from a first part of a transaction processing system, the transaction message comprising a first transaction device identifier, an encrypted transaction device identifier C and an ephemeral public key PD;
generating, at the transaction processing server, a shared secret S using a private key ds for the transaction processing server and the ephemeral public key PD;

decrypting, at the transaction processing server, the encrypted transaction device identifier C using the shared secret S to generate a second transaction device identifier ID;
processing the transaction message based on the second transaction device identifier; and verifying the transaction data using the generated original transaction device identifier ID, wherein verifying the transaction data comprises:
looking up, by using the generated original transaction device identifier ID a hashing key K for the transaction device;
recreating a hash value h' by inputting the hashing key K and a concatenation of the generated original transaction device identifier ID and transaction data for the transaction and recreating an ephemeral public key P
'D
for the transaction device using the recreated hash value h' and the cyclic group G; and comparing the ephemeral public key PD comprised in the transaction message and the recreated ephemeral public key P 'D.

14. The method of claim 13, wherein the processing comprises sending a modified transaction message with the first transaction device identifier replaced by the second transaction device identifier to a second part of a transaction processing sy stem.

15. The method of claim 13, comprising storing an association between the first transaction device identifier and the second transaction device identifier.

16. The method of claim 15, comprising:
receiving a response message from the second part of the transaction processing system, the response message comprising a data field configured to hold a transaction device identifier, the data field comprising the second transaction device identifier;
modifying the response message to replace the second transaction device identifier with the first transaction device identifier; and sending the modified response message to the first part of the transaction processing system.

17. The method of any one of claims 13 to 16, comprising using, in addition, a value uniquely associated with the second transaction device identifier, to verify the further data.

18. The method of any one of claims 13 to 17, wherein the transaction device identifier comprises data indicative of a primary account number of a financial instrument.

19. Apparatus for use in transmitting a transaction message from a transaction device having a transaction device identifier ID to a transaction processing system, wherein the transaction message conforms to an EMV standard for payment processing in which the transaction message comprises, at least, a first data field configured to hold a transaction device identifier, a second data field configured to hold supplementary data, a third data field, and other data fields for conveying transaction data, the apparatus configured to:
receive transaction data from a transaction processing server of a transaction processing system;
receive a public key Ps for the transaction processing server generated based on a cyclic group G and a private key ds for the transaction processing server;
calculate a hash value h by inputting a hashing key K, known to both the transaction device and the transaction processing server, and a concatenation of the transaction device identifier and the transaction data for a transaction;
generate an ephemeral public key PD for the transaction device using the hash value h and the cyclic group G;
generate a shared secret S using the hash value h and a public key Ps for the transaction processing server;
encrypt the transaction device identifier for the transaction device using the shared secret S to generate an encrypted transaction device identifier C;
generate the transaction message for the transaction processing system; and send the transaction message to the transaction processing system, wherein the apparatus is arranged to generate the transaction message such that data that does not identify the transaction device is provided in the first data field, the encrypted transaction device identifier C is provided in the second data field, and the ephemeral public key PD is provided in the third data field.

20. Apparatus for processing a transaction message, wherein the transaction message conforms to an EMV standard for payment processing in which the transaction message comprises, at least, a first data field configured to hold a transaction device identifier, a second data field configured to hold supplementary data, a third data field, and other data fields conveying transaction data, the apparatus configured to:
receive a transaction message from a first part of a transaction processing sy stem;
generate, at a transaction processing server, a shared secret S using a private key ds for the transaction processing server and the ephemeral public key PD
stored in the third data field;
decrypt an encrypted transaction device identifier C stored in the second data field using the shared secret S to generate an original transaction device identifier ID;
process the transaction message using the generated original transaction device identifier ID to form the transaction device identifier instead of the data in the first data field; and validate the transaction data using the generated original transaction device identifier ID, wherein validating the transaction data comprises:
looking up a hashing key K corresponding to the transaction device identifier;
obtaining a recreated hash value h ' by inputting the hashing key K and a concatenation of the generated original transaction device identifier ID and the transaction data for a transaction and obtaining a recreated ephemeral public key P 'D for the transaction device using the recreated hash value h ' and a cyclic group G; and comparing the ephemeral public key PD stored in the third data field and sent in the transaction message and the recreated ephemeral public key P

21. Apparatus for use in transmitting a transaction message from a transaction device having a transaction device identifier ID to a transaction processing system, the apparatus configured to:
receive transaction data for a transaction;
receive a public key Ps for the transaction processing server generated based on a cyclic group G and a private key ds for the transaction processing server;
calculate a hash value h by inputting a hashing key K, known to both the transaction device and the transaction processing server, and a concatenation of the transaction device identifier and the transaction data;
generate an ephemeral public key PD for the transaction device using the hash value h and the cyclic group G;
generate a shared secret S using the hash value h and a public key Ps for the transaction processing server;
encrypt the transaction device identifier ID for the transaction device using the shared secret S to generate an encrypted transaction device identifier C;
send the transaction message to the transaction processing system, the transaction message comprising, at least, the ephemeral public key PD and the encrypted transaction device identifier C.

22. Apparatus for processing a transaction message from a transaction device, the apparatus configured to:
receive the transaction message from a first part of a transaction processing system, the transaction message comprising a first transaction device identifier, an encrypted transaction device identifier C and an ephemeral public key PD; and generate a shared secret S using a private key ds for the transaction processing server and the ephemeral public key PD;
decrypt the encrypted transaction device identifier C using the shared secret S
to generate a second transaction device identifier ID;
process the transaction message based on the second transaction device identifier; and verify the transaction data using the generated original transaction device identifier ID, wherein verifying the transaction data comprises:

looking up, by using the generated original transaction device identifier ID a hashing key K for the transaction device;
recreating a hash value h' by inputting the hashing key K and a concatenation of the generated original transaction device identifier ID and transaction data for the transaction and recreating an ephemeral public key P
'D
for the transaction device using the recreated hash value h' and the cyclic group G; and comparing the ephemeral public key PD comprised in the transaction message and the recreated ephemeral public key P 'D.

23. A computer-readable medium having recorded thereon instructions to be performed by a transaction device to carry out a method of transmitting a transaction message from the transaction device, the transaction device having a transaction device identifier ID to a transaction processing system, wherein the transaction message conforms to an EMV standard for payment processing in which the transaction message comprises, at least, a first data field configured to hold a transaction device identifier, a second data field configured to hold supplementary data, a third data field, and other data fields for conveying transaction data, to the method comprising:
receiving transaction data from a transaction processing server of a transaction processing system;
receiving a public key Ps for the transaction processing server generated based on a cyclic group G and a private key ds for the transaction processing server;
calculating a hash value h by inputting a hashing key K, known to both the transaction device and the transaction processing server, and a concatenation of the transaction device identifier and the transaction data for a transaction;
generating an ephemeral public key PD for the transaction device using the hash value h and the cyclic group G;
generating a shared secret S using the hash value h and a public key Ps for the transaction processing server;
encrypting the transaction device identifier for the transaction device using the shared secret S to generate an encrypted transaction device identifier C;
generating the transaction message for the transaction processing system; and sending the transaction message to the transaction processing system, wherein the generating of the transaction message comprises providing data in the first data field of the transaction message that does not identify the transaction device, providing the encrypted transaction device identifier in the second data field of the transaction message, and providing the ephemeral public key PD in the third data field.

24. A computer-readable medium having recorded thereon instructions to be carried out by a processor to carry out a method for processing a transaction message, wherein the transaction message conforms to an EMV standard for payment processing in which the transaction message comprises, at least, a first data field configured to hold a transaction device identifier, a second data field configured to hold supplementary data, a third data field, and other data fields conveying transaction data, the method comprising steps of:
receiving a transaction message from a first part of a transaction processing sy stem;
generating a shared secret S using a private key cis for a transaction processing server and an ephemeral public key PD stored in the third data field;
decrypting an encrypted transaction device identifier C stored in the second data field using the shared secret S to generate an original transaction device identifier ID;
processing the transaction message using the generated original transaction device identifier ID to form the transaction device identifier instead of the data in the first data field; and validating the transaction data using the generated original transaction device identifier ID, wherein validating the transaction data comprises:
looking up a hashing key K corresponding to the transaction device identifier;
obtaining a recreated hash value h' by inputting the hashing key K and a concatenation of the generated original transaction device identifier ID and the transaction data for a transaction and obtaining a recreated ephemeral public key P 'D for the transaction device using the recreated hash value h' and a cyclic group G; and comparing the ephemeral public key PD stored in the third data field and sent in the transaction message and the recreated ephemeral public key P 'D.

25. A computer-readable medium having recorded thereon to be carried out by a transacting device to perform a method of transmitting a transaction message, the transaction device having a transaction device identifier ID to a transaction processing system, method comprising:
receiving transaction data for a transaction;
receiving a public key Ps for a transaction processing server generated based on a cyclic group G and a private key ds for the transaction processing server;
calculating a hash value h by inputting a hashing key K, known to both the transaction device and the transaction processing server, and a concatenation of the transaction device identifier and the transaction data;
generating an ephemeral public key PD for the transaction device using the hash value h and the cyclic group G;
generating a shared secret S using the hash value h and a public key Ps for the transaction processing server;
encrypting the transaction device identifier ID for the transaction device using the shared secret S to generate an encrypted transaction device identifier C;
sending the transaction message to the transaction processing system, the transaction message comprising, at least, the ephemeral public key PD and the encrypted transaction device identifier C.

26. A computer-readable medium having instructions recorded thereon to be performed by a processor to carry out a method for processing a transaction message from a transaction device, to the method comprising steps of:
receiving the transaction message from a first part of a transaction processing system, the transaction message comprising a first transaction device identifier, an encrypted transaction device identifier C and an ephemeral public key PD;
generating a shared secret S using a private key ds for the transaction processing server and the ephemeral public key PD;
decrypting the encrypted transaction device identifier C using the shared secret S to generate a second transaction device identifier ID;

processing the transaction message based on the second transaction device identifier; and verifying the transaction data using the generated original transaction device identifier ID, wherein verifying the transaction data comprises:
looking up, by using the generated original transaction device identifier ID a hashing key K for the transaction device;
recreating a hash value h' by inputting the hashing key K and a concatenation of the generated original transaction device identifier ID and transaction data for the transaction and recreating an ephemeral public key P
'D
for the transaction device using the recreated hash value h' and the cyclic group G; and comparing the ephemeral public key PD comprised in the transaction message and the recreated ephemeral public key P 'D.